The present invention relates to code-division multiple access (CDMA) radiotelephone communications.
CDMA is a method of spread spectrum digital communication in which a plurality of transmission channels are created by using spreading sequences for each channel that modulate the information bits to be transmitted. The spreading sequences operate at a chip rate higher than the data bit rate in order to achieve spectrum spreading of the radio signal. Their self and cross-correlation properties are adapted to enable the various channels to be multiplexed: they are generally pseudorandom sequences that are mutually orthogonal or quasi-orthogonal, taking chip values of -1 or +1.
The use of CDMA in the field of cellular radiotelephony is described in chapter I of the work "Mobile radio communications" by Raymond Steele, Pentech Press, London 1992, and also in the article "On the system design aspects of code division multiple access (CDMA) applied to digital cellular and personal communications networks" by A. Salmasi and K. S. Gilhousen, Proc. of the 41st IEEE Vehicular Technology Conference, St. Louis, Mo., May 19-22, 1991 and in U.S. Pat. No. 5,103,459. The multiplexed transmission channels are formed at the base station of each cell in the network. Each mobile station situated within the cell uses a special spreading sequence to recover, from the overall radio signal transmitter the base station, the data bits that are adressed thereto.
In the system described in the above publications, the spreading sequences are complex, i.e. each of them is made of two real pseudo-random sequences forming, respectively, the real part of the spreading sequence which modulates the bits to be transmitted in order to form an in-phase component, and the imaginary part of the spreading sequence which modulates the same bits in order to form a quadrature component.
This modulation scheme is illustrated by the diagram of FIG. 1 which shows the modulation stage of a conventional base station. In this diagram, there are shown m voice communication channels over which voice information bits D1(t), . . . , Dm(t) previously encoded by conventional techniques (block coding, interleaving . . . ) are transmitted, a service channel over which data DS(t) used for communication management are transmitted, and a pilot channel over which no information bits are transmitted.
A complex pilot sequence CP=(CP.sub.I, CP.sub.Q) used for synchronizing mobile stations and for estimating propagation paths is transmitted over the pilot channel. In the service channel, two multipliers 10,11 are used for modulating the data DS(t) by the complex spreading sequence CS=(CS.sub.I,CS.sub.Q). In each voice communication channel, two multipliers 12,13 are used for modulating the bits D1 (t), . . . , Dm(t). After a digital-to-analog conversion 14, the in-phase components relating to the various CDMA channels are summed (adder 16), as well as the quadrature components (adder 17), in order to form an in-phase signal I and a quadrature signal Q. The carrier modulation stage 20 performs quadrature phase shift keying (QPSK): two radio waves in quadrature are provided by a local oscillator 18 and a .pi./2 dephasor 19, and then modulated by the in-phase and quadrature signals I,Q by means of mixers 21,22. The two quadrature-modulated waves are summed in 23, and the resulting radio signal is subjected to bandpass filtering 24 and amplification 26 before being transmitted within the cell by the omnidirectional antenna 27.
For receiving the information bits Dk(t) that are adressed thereto, a mobile station comprises a conventional CDMA receiver, such as the one shown in the diagram of FIG. 2. The radio signal received on antenna 31 is first subjected to amplification 32 and bandpass filtering 33 before being delivered to the carrier demodulation stage 34. The latter comprises a local oscillator 36 and a .pi./2 dephasor 37 which deliver two quadrature radio waves at the carrier frequency which are used to produce an in-phase component r.sub.I and a quadrature component r.sub.Q. The first radio wave is mixed with the received signal in 38, and the resulting signal is low-pass filtered by filter 39 before being digitalized by the analog-to-digital converter 41 to provide r.sub.I. The quadrature wave is mixed with the received signal in 42, and the resulting signal is low-pass filtered by filter 43 before being digitalized by the converter 44 to provide r.sub.Q.
The receiver is usually a diversity receiver when it is desired to take advantage of several simultaneous propagation paths, as allowed for by CDMA (in this respect, see U.S. Pat. No. 5,109,390). Accordingly, a rake receiver includes a plurality of receiving arms each performing signal reception on a particular path, the results delivered by each arm being combined to improve the reception performances. Each path is detected based on the signal transmitted on the pilot channel. This procedure also enables to perform a soft handoff when the mobile station moves from one cell to another cell (see U.S. Pat. No. 5,101,501).
FIG. 2 illustrates the conventional configuration of one arm of the rake receiver (the processing applied to the service channel is not illustrated in order to simplify the figure). The pilot channel processing module 46 comprises four correlators 47 for calculating the correlations between each one of the in-phase and quadrature components r.sub.I, r.sub.Q and each one of the real and imaginary parts of the pilot sequence CP=(CP.sub.I,CP.sub.Q). The sum of the correlation between r.sub.I and CP.sub.I and of the correlation between r.sub.Q and CP.sub.Q, calculated in 48, forms the real part of a complex signal Ae.sup.j.phi. which is an estimation of the amplitude and phase response of the propagation path. The difference between the correlation between r.sub.Q and CP.sub.I and the correlation between r.sub.I and CP.sub.Q, calculated in 49, forms the imaginary part of the estimation Ae.sup.j.phi.. The complex conjugate Ae.sup.-j.phi. of this estimation, calculated in 51, is addressed to a voice communication channel processing module 52. The latter includes four correlators 47, an adder 48 and a subtractor 49 having the same arrangement as those of module 46, the correlations being calculated with the complex spreading sequence pertaining to the channel Ck=(Ck.sub.I,Ck.sub.Q). A complex multiplier 53 multiplies the complex signal output by adder 48 and subtractor 49 of module 52 by Ae.sup.-j.phi., so as to provide a real signal which is representative of the information bits transmitted over the channel. A threshold comparator 54 deduces the information bits Dk(t) from this real signal.
Regarding the uplink, from mobile stations to a base station, the usual practice is also to use complex spreading sequences and QPSK modulation.
The use of complex sequences tends to complicate the equipments in the base stations and the mobile stations. In particular, it has been shown that demodulation of the received signal involves the computation of four correlations, each of them requiring a high performance correlator, capable of performing the computations at a rate at least equal to the chip rate of the spreading sequences (typically of the order of a few megahertz). It would be desirable to propose a modulation and demodulation scheme enabling to simplify the equipments required for its implementation.
Theoretically, it is possible to use real spreading sequences and binary phase shift keying (BPSK)(see, e.g., "Theory of Spread-Spectrum Communications--A Tutorial" by R. L. Pickholtz et al, IEEE Transactions on Communications, Vol. COM-30, No. 5, May 1982) . However, such solution is not optimal as regards interchannel interferences. The residual cross-correlation between the spreading sequences (which is not exactly zero because the pseudorandom sequences are not exactly orthogonal) causes interchannel interferences which appear as a white noise in the signal obtained after the correlation. It is an advantage of the use of complex spreading sequences to reduce the level of such noise. This can be understood by considering, by way of example, the ideal transmission, over a single non-noisy path with Ae.sup.j.phi. =1, of a radio signal obtained by combining information bits D1, . . . , Dm on m CDMA channels. Then, the complex signal received by the mobile station after carrier demodulation can be expressed as: ##EQU1##
Once channel k has been processed by module 52 of FIG. 2, the estimation of a bit Dk can be expressed as (T.sub.b designating the duration of an information bit used as the integration time in the correlators 47): ##EQU2##
The last two terms represent the noise caused by the interferers. The third term is not problematic because it is purely imaginary while Dk is obtained on the real axis. In the second term, the contributions of the cross-correlations Ci.sub.I Ck.sub.I and Ci.sub.Q Ck.sub.Q are non-coherently summed when the sequences are complex with independent real and imaginary parts, while they are coherently summed when the sequences are real (Ci.sub.I =Ci.sub.Q for any i). Therefore, the use of complex sequences gives rise to a gain of about 3dB on the signal-to-interferers ratio (SIR) as compared with the use of real sequences.
That is why real spreading sequences are not used in the known CDMA systems, in spite of the substantial simplification that they would provide.